618 lines
19 KiB
Rust
618 lines
19 KiB
Rust
//! Lazy values and one-time initialization of static data.
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#[cfg(test)]
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mod tests;
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use crate::{
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cell::{Cell, UnsafeCell},
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fmt,
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marker::PhantomData,
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mem::MaybeUninit,
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ops::{Deref, Drop},
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panic::{RefUnwindSafe, UnwindSafe},
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pin::Pin,
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sync::Once,
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};
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#[doc(inline)]
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#[unstable(feature = "once_cell", issue = "74465")]
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pub use core::lazy::*;
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/// A synchronization primitive which can be written to only once.
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///
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/// This type is a thread-safe `OnceCell`.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// static CELL: SyncOnceCell<String> = SyncOnceCell::new();
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/// assert!(CELL.get().is_none());
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///
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/// std::thread::spawn(|| {
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/// let value: &String = CELL.get_or_init(|| {
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/// "Hello, World!".to_string()
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/// });
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/// assert_eq!(value, "Hello, World!");
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/// }).join().unwrap();
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///
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/// let value: Option<&String> = CELL.get();
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/// assert!(value.is_some());
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/// assert_eq!(value.unwrap().as_str(), "Hello, World!");
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub struct SyncOnceCell<T> {
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once: Once,
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// Whether or not the value is initialized is tracked by `state_and_queue`.
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value: UnsafeCell<MaybeUninit<T>>,
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/// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
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///
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/// ```compile_fail,E0597
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// struct A<'a>(&'a str);
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///
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/// impl<'a> Drop for A<'a> {
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/// fn drop(&mut self) {}
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/// }
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///
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/// let cell = SyncOnceCell::new();
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/// {
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/// let s = String::new();
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/// let _ = cell.set(A(&s));
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/// }
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/// ```
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_marker: PhantomData<T>,
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}
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// Why do we need `T: Send`?
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// Thread A creates a `SyncOnceCell` and shares it with
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// scoped thread B, which fills the cell, which is
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// then destroyed by A. That is, destructor observes
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// a sent value.
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#[unstable(feature = "once_cell", issue = "74465")]
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unsafe impl<T: Sync + Send> Sync for SyncOnceCell<T> {}
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#[unstable(feature = "once_cell", issue = "74465")]
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unsafe impl<T: Send> Send for SyncOnceCell<T> {}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}
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#[unstable(feature = "once_cell", issue = "74465")]
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#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
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impl<T> const Default for SyncOnceCell<T> {
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/// Creates a new empty cell.
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///
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/// # Example
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// fn main() {
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/// assert_eq!(SyncOnceCell::<()>::new(), SyncOnceCell::default());
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/// }
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/// ```
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fn default() -> SyncOnceCell<T> {
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SyncOnceCell::new()
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: fmt::Debug> fmt::Debug for SyncOnceCell<T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self.get() {
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Some(v) => f.debug_tuple("Once").field(v).finish(),
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None => f.write_str("Once(Uninit)"),
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}
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Clone> Clone for SyncOnceCell<T> {
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fn clone(&self) -> SyncOnceCell<T> {
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let cell = Self::new();
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if let Some(value) = self.get() {
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match cell.set(value.clone()) {
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Ok(()) => (),
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Err(_) => unreachable!(),
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}
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}
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cell
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T> From<T> for SyncOnceCell<T> {
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/// Create a new cell with its contents set to `value`.
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///
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/// # Example
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// # fn main() -> Result<(), i32> {
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/// let a = SyncOnceCell::from(3);
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/// let b = SyncOnceCell::new();
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/// b.set(3)?;
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/// assert_eq!(a, b);
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/// Ok(())
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/// # }
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/// ```
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fn from(value: T) -> Self {
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let cell = Self::new();
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match cell.set(value) {
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Ok(()) => cell,
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Err(_) => unreachable!(),
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}
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: PartialEq> PartialEq for SyncOnceCell<T> {
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fn eq(&self, other: &SyncOnceCell<T>) -> bool {
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self.get() == other.get()
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Eq> Eq for SyncOnceCell<T> {}
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impl<T> SyncOnceCell<T> {
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/// Creates a new empty cell.
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#[unstable(feature = "once_cell", issue = "74465")]
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#[must_use]
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pub const fn new() -> SyncOnceCell<T> {
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SyncOnceCell {
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once: Once::new(),
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value: UnsafeCell::new(MaybeUninit::uninit()),
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_marker: PhantomData,
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}
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}
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/// Gets the reference to the underlying value.
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///
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/// Returns `None` if the cell is empty, or being initialized. This
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/// method never blocks.
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get(&self) -> Option<&T> {
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if self.is_initialized() {
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// Safe b/c checked is_initialized
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Some(unsafe { self.get_unchecked() })
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} else {
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None
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}
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}
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/// Gets the mutable reference to the underlying value.
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///
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/// Returns `None` if the cell is empty. This method never blocks.
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_mut(&mut self) -> Option<&mut T> {
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if self.is_initialized() {
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// Safe b/c checked is_initialized and we have a unique access
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Some(unsafe { self.get_unchecked_mut() })
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} else {
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None
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}
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}
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/// Sets the contents of this cell to `value`.
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///
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/// May block if another thread is currently attempting to initialize the cell. The cell is
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/// guaranteed to contain a value when set returns, though not necessarily the one provided.
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///
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/// Returns `Ok(())` if the cell's value was set by this call.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// static CELL: SyncOnceCell<i32> = SyncOnceCell::new();
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///
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/// fn main() {
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/// assert!(CELL.get().is_none());
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///
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/// std::thread::spawn(|| {
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/// assert_eq!(CELL.set(92), Ok(()));
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/// }).join().unwrap();
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///
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/// assert_eq!(CELL.set(62), Err(62));
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/// assert_eq!(CELL.get(), Some(&92));
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/// }
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn set(&self, value: T) -> Result<(), T> {
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let mut value = Some(value);
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self.get_or_init(|| value.take().unwrap());
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match value {
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None => Ok(()),
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Some(value) => Err(value),
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}
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}
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/// Gets the contents of the cell, initializing it with `f` if the cell
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/// was empty.
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///
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/// Many threads may call `get_or_init` concurrently with different
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/// initializing functions, but it is guaranteed that only one function
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/// will be executed.
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///
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/// # Panics
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///
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/// If `f` panics, the panic is propagated to the caller, and the cell
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/// remains uninitialized.
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///
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/// It is an error to reentrantly initialize the cell from `f`. The
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/// exact outcome is unspecified. Current implementation deadlocks, but
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/// this may be changed to a panic in the future.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// let cell = SyncOnceCell::new();
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/// let value = cell.get_or_init(|| 92);
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/// assert_eq!(value, &92);
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/// let value = cell.get_or_init(|| unreachable!());
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/// assert_eq!(value, &92);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_or_init<F>(&self, f: F) -> &T
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where
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F: FnOnce() -> T,
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{
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match self.get_or_try_init(|| Ok::<T, !>(f())) {
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Ok(val) => val,
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}
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}
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/// Gets the contents of the cell, initializing it with `f` if
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/// the cell was empty. If the cell was empty and `f` failed, an
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/// error is returned.
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///
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/// # Panics
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///
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/// If `f` panics, the panic is propagated to the caller, and
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/// the cell remains uninitialized.
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///
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/// It is an error to reentrantly initialize the cell from `f`.
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/// The exact outcome is unspecified. Current implementation
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/// deadlocks, but this may be changed to a panic in the future.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// let cell = SyncOnceCell::new();
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/// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
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/// assert!(cell.get().is_none());
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/// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
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/// Ok(92)
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/// });
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/// assert_eq!(value, Ok(&92));
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/// assert_eq!(cell.get(), Some(&92))
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
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where
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F: FnOnce() -> Result<T, E>,
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{
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// Fast path check
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// NOTE: We need to perform an acquire on the state in this method
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// in order to correctly synchronize `SyncLazy::force`. This is
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// currently done by calling `self.get()`, which in turn calls
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// `self.is_initialized()`, which in turn performs the acquire.
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if let Some(value) = self.get() {
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return Ok(value);
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}
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self.initialize(f)?;
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debug_assert!(self.is_initialized());
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// SAFETY: The inner value has been initialized
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Ok(unsafe { self.get_unchecked() })
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}
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/// Internal-only API that gets the contents of the cell, initializing it
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/// in two steps with `f` and `g` if the cell was empty.
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///
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/// `f` is called to construct the value, which is then moved into the cell
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/// and given as a (pinned) mutable reference to `g` to finish
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/// initialization.
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///
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/// This allows `g` to inspect an manipulate the value after it has been
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/// moved into its final place in the cell, but before the cell is
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/// considered initialized.
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///
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/// # Panics
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///
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/// If `f` or `g` panics, the panic is propagated to the caller, and the
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/// cell remains uninitialized.
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///
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/// With the current implementation, if `g` panics, the value from `f` will
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/// not be dropped. This should probably be fixed if this is ever used for
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/// a type where this matters.
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///
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/// It is an error to reentrantly initialize the cell from `f`. The exact
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/// outcome is unspecified. Current implementation deadlocks, but this may
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/// be changed to a panic in the future.
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pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
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where
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F: FnOnce() -> T,
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G: FnOnce(Pin<&mut T>),
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{
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if let Some(value) = self.get_ref().get() {
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// SAFETY: The inner value was already initialized, and will not be
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// moved anymore.
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return unsafe { Pin::new_unchecked(value) };
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}
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let slot = &self.value;
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// Ignore poisoning from other threads
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// If another thread panics, then we'll be able to run our closure
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self.once.call_once_force(|_| {
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let value = f();
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// SAFETY: We use the Once (self.once) to guarantee unique access
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// to the UnsafeCell (slot).
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let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
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// SAFETY: The value has been written to its final place in
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// self.value. We do not to move it anymore, which we promise here
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// with a Pin<&mut T>.
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g(unsafe { Pin::new_unchecked(value) });
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});
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// SAFETY: The inner value has been initialized, and will not be moved
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// anymore.
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unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
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}
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/// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
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/// `None` if the cell was empty.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// let cell: SyncOnceCell<String> = SyncOnceCell::new();
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/// assert_eq!(cell.into_inner(), None);
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///
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/// let cell = SyncOnceCell::new();
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/// cell.set("hello".to_string()).unwrap();
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/// assert_eq!(cell.into_inner(), Some("hello".to_string()));
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn into_inner(mut self) -> Option<T> {
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self.take()
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}
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/// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
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///
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/// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.
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///
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/// Safety is guaranteed by requiring a mutable reference.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncOnceCell;
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///
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/// let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
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/// assert_eq!(cell.take(), None);
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///
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/// let mut cell = SyncOnceCell::new();
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/// cell.set("hello".to_string()).unwrap();
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/// assert_eq!(cell.take(), Some("hello".to_string()));
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/// assert_eq!(cell.get(), None);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn take(&mut self) -> Option<T> {
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if self.is_initialized() {
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self.once = Once::new();
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// SAFETY: `self.value` is initialized and contains a valid `T`.
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// `self.once` is reset, so `is_initialized()` will be false again
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// which prevents the value from being read twice.
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unsafe { Some((&mut *self.value.get()).assume_init_read()) }
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} else {
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None
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}
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}
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#[inline]
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fn is_initialized(&self) -> bool {
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self.once.is_completed()
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}
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#[cold]
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fn initialize<F, E>(&self, f: F) -> Result<(), E>
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where
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F: FnOnce() -> Result<T, E>,
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{
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let mut res: Result<(), E> = Ok(());
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let slot = &self.value;
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// Ignore poisoning from other threads
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// If another thread panics, then we'll be able to run our closure
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self.once.call_once_force(|p| {
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match f() {
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Ok(value) => {
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unsafe { (&mut *slot.get()).write(value) };
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}
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Err(e) => {
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res = Err(e);
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// Treat the underlying `Once` as poisoned since we
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// failed to initialize our value. Calls
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p.poison();
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}
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}
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});
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res
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}
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/// # Safety
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///
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/// The value must be initialized
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unsafe fn get_unchecked(&self) -> &T {
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debug_assert!(self.is_initialized());
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(&*self.value.get()).assume_init_ref()
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}
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/// # Safety
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///
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/// The value must be initialized
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unsafe fn get_unchecked_mut(&mut self) -> &mut T {
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debug_assert!(self.is_initialized());
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(&mut *self.value.get()).assume_init_mut()
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}
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}
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unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
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fn drop(&mut self) {
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if self.is_initialized() {
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// SAFETY: The cell is initialized and being dropped, so it can't
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// be accessed again. We also don't touch the `T` other than
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// dropping it, which validates our usage of #[may_dangle].
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unsafe { (&mut *self.value.get()).assume_init_drop() };
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}
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}
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}
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|
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/// A value which is initialized on the first access.
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///
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/// This type is a thread-safe `Lazy`, and can be used in statics.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::collections::HashMap;
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///
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/// use std::lazy::SyncLazy;
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///
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/// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| {
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/// println!("initializing");
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/// let mut m = HashMap::new();
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/// m.insert(13, "Spica".to_string());
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/// m.insert(74, "Hoyten".to_string());
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/// m
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/// });
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///
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/// fn main() {
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/// println!("ready");
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/// std::thread::spawn(|| {
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/// println!("{:?}", HASHMAP.get(&13));
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/// }).join().unwrap();
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/// println!("{:?}", HASHMAP.get(&74));
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///
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/// // Prints:
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/// // ready
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/// // initializing
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/// // Some("Spica")
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/// // Some("Hoyten")
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/// }
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub struct SyncLazy<T, F = fn() -> T> {
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cell: SyncOnceCell<T>,
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init: Cell<Option<F>>,
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: fmt::Debug, F> fmt::Debug for SyncLazy<T, F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive()
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}
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}
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// We never create a `&F` from a `&SyncLazy<T, F>` so it is fine
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// to not impl `Sync` for `F`
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// we do create a `&mut Option<F>` in `force`, but this is
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// properly synchronized, so it only happens once
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// so it also does not contribute to this impl.
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#[unstable(feature = "once_cell", issue = "74465")]
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unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {}
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// auto-derived `Send` impl is OK.
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T, F: UnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T, F: UnwindSafe> UnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: UnwindSafe {}
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impl<T, F> SyncLazy<T, F> {
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/// Creates a new lazy value with the given initializing
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/// function.
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#[unstable(feature = "once_cell", issue = "74465")]
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pub const fn new(f: F) -> SyncLazy<T, F> {
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SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) }
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}
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}
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impl<T, F: FnOnce() -> T> SyncLazy<T, F> {
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/// Forces the evaluation of this lazy value and
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/// returns a reference to result. This is equivalent
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/// to the `Deref` impl, but is explicit.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::SyncLazy;
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///
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/// let lazy = SyncLazy::new(|| 92);
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///
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/// assert_eq!(SyncLazy::force(&lazy), &92);
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/// assert_eq!(&*lazy, &92);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn force(this: &SyncLazy<T, F>) -> &T {
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this.cell.get_or_init(|| match this.init.take() {
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Some(f) => f(),
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None => panic!("Lazy instance has previously been poisoned"),
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})
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T, F: FnOnce() -> T> Deref for SyncLazy<T, F> {
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type Target = T;
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fn deref(&self) -> &T {
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SyncLazy::force(self)
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Default> Default for SyncLazy<T> {
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/// Creates a new lazy value using `Default` as the initializing function.
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fn default() -> SyncLazy<T> {
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SyncLazy::new(T::default)
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}
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}
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